chapter 5

34 Pages

## Combinatorial Problems in Algorithmic Differentiation

Conceptually, in Algorithmic (also known as Automatic) Differentiation (AD) we consider multivariate vector functions F : Rn → Rm mapping a vector of independent inputs x ∈ Rn onto a vector of dependent outputs y ∈ Rm as y = F (x). For the purpose of the current chapter the function F is assumed to be twice continuously differentiable in a neighborhood of all

arguments at which it is supposed to be evaluated. Hence, its Jacobian

∇F (x) = ( ∂yj ∂xi

∈ Rm×n

and Hessian

∇2F (x) = (

∂2yj ∂xi∂xk

∈ Rm×n×n

exist at all of these points. The Hessian ∇2F (x) ≡ H = (hj,i,k)j=1,...,mi,k=1,...,n is a symmetric 3-tensor, that is, hj,i,k = hj,k,i.